A tale of 2 BMIs: thoughts on redefining obesity

I am now the same weight that I was in grade 12: 154 lbs. At 5′ 11″ that gives me a body mass index (BMI) of 21.5 (it was 29 a year and a half ago). While my weight and height are the same as 17-year-old Jen, my size is not. I was a size 12 then and am a size 8 now.

It’s a good lesson in the concept that not all weight is equal. In high school I never exercised on purpose (although I did walk 4 miles a day to and from school) and was fleshy and jiggly in that youthful way. If asked to run, I would have made a face and then been severely winded at half a mile. Now I exercise aerobically on a regular basis and have a higher protein diet. As a result, I have more muscle and less fat, hence the smaller size. I am much healthier, even though I weigh exactly the same as I did 28 years ago.

Which brings me to an article published last week on redefining BMI (Shah and Braverman, April 2012 PLosOne). The investigators, by measuring adipose tissue with DXA scans, found that 40% of people in the overweight range (BMI 25-29), were actually obese and therefore at greater health risk than thought using BMI alone.

While BMI is notoriously inaccurate, this study does not tell us that if you have a BMI of 35 that perhaps you aren’t obese. What this study tells us that at the lighter end of the scale things may not always be what they seem.

So what should you think/do if you are in the 25-29 BMI range? Well, I didn’t need a DXA scan to confirm anything. When I bent forward I could grab more than 2 generous handfuls of belly fat (remember the old, “Can you pinch an inch?). I was also out of shape aerobically and wearing a size 16.

Unfortunately, DXA scans and measuring leptin levels (also used in the article) might not be practical or cost effective on a large scale. But asking about exercise, measuring waist circumference, blood pressure, glucose levels, and cholesterol levels are all good measurements to help put BMI in perspective.

Whether BMI should be abandoned altogether is an entirely different discussion (studies do link BMI of 30 and up with negative health outcomes). However, this study confirms what my being both a size 12 and a size 8 at a BMI of 21.5 tells me. That notallweightisequal, because 45-year- old Jen would kick 17-year-old Jen’s butt in not only a race, but in every measure of cardiovascular fitness.

Medical professionals should not just stop at BMI, but rather gather more data points to put weight in perspective. For everyone. Because weight is only one measurement of health.

Discussion

7 thoughts on “A tale of 2 BMIs: thoughts on redefining obesity”

What’s interesting is that, although I am at the same weight now that I was at 17, after 6 babies and losing about 58 pounds, my fat is proportioned differently. My uterus seems permanently distended from the huge (9lbs. 10 ozs.), 10-day overdue baby that was my second to last. My boobs feel bigger as well, and I breastfed all 6 of my babies – the last four for about 1 year + each. And I, like you, am in much better health physically. I could lose another 10 lbs. (and would like to), but I do take some comfort in that my BMI is in a healthy range. I think you’re right, though, that having a healthy BMI is only part of the health equation if vigorous physical exercise isn’t part of a woman’s daily life.

You also need to keep in mind that the measurements of a size 12 and size 8 have changed in the intervening years. Although there is variation among manufacturers, a size 12 in 1984 is approximately equivalent to a size 8 today.

Hmmm, good point. Although, I do have a bikini and a couple of pieces that I kept from 1984 and they are big on me now. But size inflation (or deflation) is a big issue. One son has a BMI at the 50th percentile, so exactly average, and he needs to wear the “slim” pants. Regular size falls off of him.

I find 154 hard to believe. I am 5’10”. At 175lbs I am slim trim and fit and look great in size 12 jeans. Any thinner and my bones would protrude in a grotesque way. However, when I had a bone density test at 60 the tech said I had the densest bones she had ever seen so maybe that’s a factor.